How to add Facebook comment box at bottom of each pages/posts ?

You could try this plugin that automatically replaces the standard WP comment box with a Facebook comment box. You could also try any plugins from this search.

Thank you grantkessler.

No problem, glad that I could help!

You can also use the Facebook Developers site. https://developers.facebook.com/docs/plugins/comments/

Yes the Facebook Developers website offers a bunch of neat things but the plugin above will change comment boxes automatically.